Clathrate liquids form ice at temperatures above 0.degree. C. and have been proposed as a thermal energy storage material. Clathrate ice is composed of water and a second component whose molecules are enclosed by water. Clathrates typically form clathrate ice at temperatures of 39.degree. F. or above depending on the clathrate material. The second component may be a gas such as freon or a liquid such as tetrahydrofuran. As clathrates permit ice to be formed at higher temperatures, the use of cooling temperatures above 0.degree. C. is possible.
One of the difficulties in utilizing a clathrate hydrate as a thermal energy storage material is the significant supercooling the clathrate experiences before it solidifies. This supercooling usually occurs 10 to 0.degree. C. below the clathrate freezing point.
To solve this problem, it has been proposed in Japanese Patent No. 54-102297 that two combined clathrates be used. According to the disclosure of this patent, one clathrate has a higher melting point so as to keep its solid state through the usage condition. At the same time, this clathrate works as a seeding agent for crystallization of another clathrate and thereby supresses supercooling. However, there are a number of drawbacks to such a system. In the first instance, the seeding agent may be readily dissolved in the other clathrate and thereby lose the nucleation effect after a period of time. In addition, where a porous partition lies between the seeding agent and the main thermal energy storage clathrate, the expansion force of the solidification inside of the cavities may destroy the porous partition. The inherent toxicity of the seeding agent clathrate is also a disadvantage.
There has also been disclosed in the prior art the concept of using seed crystals to increase ice formation in an otherwise supercooled liquid. According to this prior art, a piece of ice in an ice maker is retained in the mold to enhance crystallization and seeding during a succeeding freezing operation. Devices of this type are disclosed in the following U.S. Pat. Nos . 4,059,970 (Loeb); 4,062,201 (Schumacher); and 4,261,102 (Elliott).